#define _CRT_SECURE_NO_WARNINGS
#include<stdio.h>
int main()
{
	int k, past, n, day, sum;
	n = 1;
	sum = 0;
	past = 0;
	scanf("%d", &k);
	for (day = 1; day <= k; day++)
	{

	
		sum += n;
		if (day - past >= n)
		{
			past += n;
			n++;
		
		}
	}

		printf("%d", sum);

		return 0;
	}